The invention relates to a printing method and arrangement.
More particularly, the invention relates to a color printing method and arrangement, especially a method of and an arrangement for imprinting an image carrier with two or more colors which are superimposed in order to produce colors representing mixtures of the original colors.
A known arrangement for imprinting an image carrier with two or more superimposed colors has a plurality of printing stations. Each of the printing stations has a printing drum or a register which applies a different color to the image carrier. In order for the colors to be superimposed, the image carrier must have a predetermined orientation or alignment relative to each of the printing drums. To this end, a regulating mechanism is provided to ensure proper alignment of the image carrier relative to the drums.
One procedure employed for proper relative alignment of the image carrier and a drum is the so-called longitudinal alignment procedure. In this procedure, a first series of marks is formed on the image carrier by a first printing device while a second series of corresponding marks is formed on the image carrier by a second printing device. The respective series of marks lie on scanning lines which are located close to one another. Each of the scanning lines is continuously illuminated by a small light source. The light reflected from the image carrier along each of the scanning lines travels to a respective photocell. When a mark passes by a light source, the intensity of the reflected light changes. The corresponding photocell transforms the intensity change into an electrical pulse. The pulse is amplified in an electronic device in order to convert it into a form which is suitable for measuring purposes as well as for the performance of regulating functions.
The electronic device receives a separate pulse for each of two corresponding marks on the two scanning lines. The time difference between arrival of the pulses at the output of the electronic device regulates an electric current having a magnitude which depends upon the magnitude of the time difference. If, for example, two corresponding marks are widely spaced as considered in a direction along the scanning lines, the time difference between the pulses is large and the current due to this time difference is also large. The current controls an alignment motor which is arranged to move a printing drum mounted in the printing arrangement for the image carrier. The amount of movement of the printing drum depends upon the time difference between the pulses. For instance, if the time difference is large, the drum is moved by a large amount. It is possible to design the alignment motor so that it operates in forward or reverse depending upon which of two corresponding marks is detected first.
Another procedure for aligning the image carrier relative to a printing drum is the so-called lateral alignment procedure. This procedure is used for lateral alignment of the image carrier relative to the printing drum. In the lateral alignment procedure, a series of marks is imprinted on the image carrier along a line which is inclined to the direction of travel or the longitudinal direction of the image carrier. Normally, such line is inclined at an angle of 45.degree. to the direction of travel of the image carrier.
The marks for the longitudinal alignment procedure, that is, the marks which are used to align the image carrier along its longitudinal direction, may be provided in addition to those for the lateral alignment procedure. Since the scanning lines defined by the marks for the longitudinal alignment procedure extend longitudinally of the image carrier, the line defined by the marks for the lateral alignment procedure is inclined with respect to these scanning lines.
When marks for both the longitudinal and lateral alignment procedures are present, a correspondence may exist between each mark for the lateral alignment procedure and each pair of marks for the longitudinal alignment procedure. By means of an appropriate electronic circuit, two electrical pulses are generated for each set of marks consisting of a lateral alignment mark and a pair of longitudinal alignment marks. One such pulse serves to regulate longitudinal alignment of the image carrier relative to a printing drum while the other pulse serves to regulate lateral alignment of the image carrier relative to the drum. The image carrier and the printing drum are aligned relative to one another longitudinally of the image carrier in the manner described above. On the other hand, relative lateral alignment of the image carrier and the drum is effected via an alignment motor which is connected to a paper roller. This roller guides the image carrier to one side or the other.
The preceding procedures for relative alignment of an image carrier and a printing drum have certain disadvantages. To begin with, the marks must be properly positioned relative to the image or images at the same time that the carrier is produced. This, however, can be accomplished with relatively good precision only by skilled workers. Furthermore, the use of marks is based on the assumption that the positions of the marks relative to the image or images remain the same so that alignment of the marks results in proper alignment of the image carrier relative to a printing drum. This assumption ignores the fact that deformation of the image carrier is non-uniform, e.g. an area with an image may deform differently than an area with no image. Thus, in practice, the differing characteristics of an image carrier cannot be taken into account by marks. Even if the marks are precisely aligned relative to a printing drum, there is no assurance the the region of the image carrier to be imprinted is aligned in the same manner.
Anothe drawback of the alignment procedures described earlier resides in that the image carrier is only indirectly affected by the alignment operations. For example, in the longitudinal alignment procedure, the position of a printing drum is adjusted so as to conform to the orientation of the image carrier. In the lateral alignment procedure, on the other hand, one or more paper rollers are adjusted in such a manner as to guide the image carrier beneath a printing drum in the desired position. Since the image carrier is not acted on directly, it is uncertain whether the image carrier retains the position it has prior to adjustment of the printing roller or the paper roller. Depending upon the elastic properties of the image carrier, it is possible for the latter to deform in an undesired manner even when it is stressed, e.g. by a paper roller, so that uncontrolled deformation would not be expected. By way of example, a possible result of unexpected deformation is that the different colors applied by different printing drums are imprinted next to one another rather than one over the other as desired. The different colors then do not combine to form a new color representing a mixture of the different colors and the image instead contains a set of discrete lines which are arranged next to one another and have various colors.
The preceding description of the drawbacks arising from the use of marks to align an image carrier and a printing drum relative to one another indicates that the marks are useful in achieving alignment of the various printing drums of a printing arrangement with respect to each other. On the other hand, it is likewise clear that the marks are not suitable as an aid in obtaining proper relative alignment of an image and a printing drum.
In certain conventional printing arrangements, the intervals at which consecutive marks along a row pass by a photocell are measured. Depending on the lengths of the intervals, the positions of selected components of such a printing arrangement are changed in such a manner as to permit adjustment of the position of the image carrier which may, for instance, be constituted by a paper band.
In a printing arrangement of this type, the intervals between consecutive marks may be precisely determined. By regulating appropriate components of the printing arrangement in dependence upon the measured intervals, the intervals between consecutive marks are maintained substantially constant.
Since a constant interval between consecutive marks is not critical to obtaining proper relative alignment of an image carrier and a printing drum, the equipment for maintaining the intervals between consecutive marks constant increases the complexity and cost of the printing arrangement unnecessarily.